1. Field of the Invention
The present invention relates to a driving method for an electric field electron emission apparatus.
2. Background Art
It is known that electron can be emitted efficiently from a carbon material called xe2x80x9ccarbonnanotubesxe2x80x9d. Efficient electron emission means herein that electrons are emitted by a low electric field and at a low threshold value.
Measurement of the electric field electron emission of the carbon nanotube yields the characteristic curve shown in FIG. 6. The measurement is carried out by applying a positive voltage to the anode electrode, located 1 mm apart from and facing a small piece of the carbon nanotube, which is fixed on a ground metallic cathode electrode. The result of the measurement is obtained by increasing the anode voltage from the ground potential to a certain maximum voltage and then decreasing from the maximum voltage to the ground potential.
In the above measurement, the anode voltage is increased and decreased at a rate of 10V per second. It was found in the measurement that the rate of increase of the electron current density tends to become saturated, and the region where the rate of increase of the electron current density tends to become saturated is referred to as the saturation tendency region. In this saturation tendency region, it was observed that the amount of emitted electrons is reduced during the application of a voltage. The above-described reduction means that the threshold of electron emission becomes higher, or that the amount of emitted electron decreases even when the same electric field is applied.
Since the electric potentials of both cathode and anode electrodes and the distance between the cathode and anode electrodes varies depending upon the type of the measurement apparatus, the voltage applied to the cathode electrode is called the electric field in this application. However, since a thin electron emitter is attached to the cathode electrode, the voltage applied to the cathode electrode can be used for expressing the potential bias between the electrode and the electron emitter.
When applying the electric field in the saturation tendency region, the reduction of the amount of emitted electrons becomes smaller as duration of application of the same electric field is shorter. The degradation of the electron emission characteristic becomes greater when the period of application of the voltage is the same but the electric field is higher. In contrast, when the electric field is below the saturation tendency region, the electron emission characteristic is very small and insignificant. As indicated above, the problem has been found that the electron emission characteristics are degraded in the saturation tendency region.
The degradation of the electron emission characteristics in the saturation tendency region is also observed in various materials. For example, it was found in a micro-filed emitter array using a spinto-type molybdenum cone that its electron emission characteristics show a tendency to saturate significantly departing from the Fauler-Nordheim function. In this saturation tendency region, it was observed that the electron emission characteristics are remarkably degraded. The same degradation was observed in DLC (diamond like carbon) and in the DLC material treated by hydrogen plasma or the like.
It is therefore the object of the present invention to provide a method first for driving an electric field electron emission apparatus which emits electron by electric field emission from the cathode electrode and captures electrons at the anode electrode, wherein the electric field applied to the cathode electrode is higher than that for generating the field emission and lower than the electric field in which the second derivative of the density of the electron current (hereinafter called the electron current density) to be captured at the cathode electrode with respect to the cathode electrode becomes 0 for the first time after the electric field is applied.
In this case, it is possible to prevent degradation of the electron emission characteristic by not driving the electron emission apparatus in the saturation tendency region, by mathematically recognizing that the electric field is in the saturation tendency region of the electron emission characteristic. In the normal electron emission region, the amount of emitted electrons increases faster than the increase of the applied field, that is, the second derivative of the electron emission increases in the positive region.
In the second method of driving the electron emission apparatus, in which electrons are emitted from the cathode electrode and are captured at the anode electrode, wherein the electric field applied to said cathode electrode is higher than the electric field for generating the field emission of electrons, and the second derivative of the density of the electron current (hereinafter, called the electron current density) captured by the anode electrode for the electric field is higher than the electric field in which the second derivative becomes 0 for the first time after the electric field is applied, and wherein the period of time during which the electric field is applied to the cathode electrode satisfies the following two equations:
tap=T/|A|
Estxe2x89xa6E
where,
T: a numeral value within a range higher than 1xc3x9710xe2x88x929 and lower than 1xc3x9710xe2x88x926 [secxc2x7cmxe2x88x922xc2x7Vxe2x88x922xc2x7A],
A: the second derivative of the electron current density [Axc2x7cmxe2x88x922xc2x7Vxe2x88x922],
E: the electric field applied to the cathode electrode, and
Est: the minimum electric field in a saturation tendency region,
wherein, the minimum electric field in the saturation tendency region is the electric field at which a change of the electron current density changes from an increase to a decrease.